We propose to carry out several projects directed toward obtaining a more complete understanding of the mechanisms of solute and water transport across the pulmonary microvascular endothelium and alveolar epithelium. We are particularly interested in determining the characteristics of the pores thought to exist in these membranes, how those characteristics change during lung development and in response to certain chemical challenges, and how freely certain important molecules cross these membranes. Using isolated, perfused rabbit lungs and techniques we have previously employed, we intend, whenever possible, to collect data from both osmotic and permeability studies so as to obtain a more complete picture of these membranes than is currently available. Specifically, we intend to measure the fluid filtration coefficient, solute reflection coefficients, and sole permeability coefficients, and to use this information in conjunction with a new theory to express the porosity of each membrane in terms of a continuous distribution of pore sizes. Once this has been done for the normal, adult lung, the same procedures will be employed to investigate fetal, newborn and immature lungs, lungs treated with endotoxins and suspected mediators of inflammation, and the transport of elastase, protease and related substances known to have important pulmonary effects.